1. Field of the Invention
The present invention is related to dynamometer machines and particularly to improvements in automatic weighing machines, wherein the load to be weighed is placed on a platform of the weighing machine, and a weight indication or reading independent of the load's relative position may be obtained. The present invention is particularly related to automatic weighing machines, i.e. those of the dynamometer type giving a read-out automatically upon a load being placed on their platform.
The type of weighing machine referred to herein, generally comprises a pair of side-by-side symmetrical beams lodged between a frame and a tray or platform, the latter being adapted to receive and support different weight loads. The beams are held in place at each end portion thereof by pairs of essentially vertical coupling members connected to transverse load the beam at each end portion in response to a weight load on the platform; which pairs of coupling members comprise, connected to each end portion of each beam, one member supporting the platform to transmit a load force, and another member similarly resting on the frame to transmit a reactive force dependent on load parameters. These load parameters are weight magnitude and eccentricity; the latter is defined by the position of the load in relation to the platform. The transverse loading of the beams creates bending moments at a middle portion thereof due to the longitudinal offset between the respective coupling points where the coupling members apply the load and the reactive forces to each end portion of the beams.
2. Description of the Prior Art
It is already known in the art that an accurate weight read-out may be obtained regardless of the relative position of the load on the platform of a weighing machine, i.e. even when the eccentricity of the point of application of the resultant force applied by the load on the platform is great. U.S. Pat. No. 2,899,191 explains that the sum of the bending moments at the longitudinal centre of each beam may vary only according to the magnitude of the weight, and is independent of the eccentricity of the load on the receiving platform. The centre of each symmetrical beam may thus be considered an invariant sensing position insofar the relative position of the load. Strain-gauges are attached to the beam at its invariant centre position, to detect the bending moment thereat. Argentine Pat. No. 216,822 shows means coupling the straight flat beams between the platform and the frame, adequate for transverse loading the beams without substantially subjecting them to other deforming moments.
In these types of weighing machines, the transfer function relating the weight of the load with the bending moment at the centre of each beam depends on geometrical factors which are generally assumed to be constant, such as the longitudinal component of the distance between the point supporting the platform and the point resting on the frame, at each end of the beam. In this sense, Argentine Pat. No. 216,822 also shows how a weighing machine may be manufactured, in which this distance is uniquely determined by the geometry of the beams. However, for different flexures of the beams, both these points move unequal trajectories, resulting in that said transfer function varies under different load conditions, giving rise to a so-called angle error which limits the precision of the machine.
Another source of error arises from the effect of horizontal forces, which are evidently disturbing, because weight is an inherently vertical vector parameter. Thus, any bending moment components due to non-vertical forces (i.e. horizontal force components) evidently introduce errors in the overall weight measurement. As explained more fully hereinafter, horizontal forces are originated by various sources: (a) inclination of the forces transmitted by the coupling members to the beams, with respect to the vertical direction; (b) friction between the beams and the coupling members during flexure, because of relative longitudinal movement between these means; and (c) elasticity of the coupling members as they yield under load. Due to the finite thickness of the beams, these horizontal forces are vertically offset and consequently contribute an error factor to the final bending moment at the centre of each beam. This is called the bending moment error. For all these reasons, the resolution of this type of prior art machine is limited to less than 1:1,000.
In large weighing machines, all these deficiencies are further magnified, because of the greater deflection of the centre of longer beams.